Study On Precise Measurement Of Silicon Density Difference By Pressure Floating Method

Density as a derived physical quantity of the measurement fields,is an essential measure physical quantity in international trade and people's livelihood.Silicon crystal is the best choice for the establishment of solid density reference,while the first choice for the calculation of Avogadro NA is also silicon crystal.In this paper,we design a precision measurement system of silicon density by the pressure floating method.The precise measurement of silicon density difference device can accurately measure the density difference between silicones;provide a basis for the calculation of Avogadro NA.The main contents of this paper are as follows:1.The present situation of silicon density measurement at home and abroad is reviewed,analyzing the advantages and disadvantages of three kinds of density difference measuring methods: the hydrostatic weighing method,the magnetic suspension method and the pressure floating method.The Pressure Floating Method(PFM)is used to measure the density of silicon,providing a theoretical basis for the precise measurement of the density difference of single crystal silicones.2.The measuring principle of the system is analyzed,and the device for measuring the silicon spheres density difference is designed,including: density comparison vessel,temperature control system,pressure control system,charge coupled device(CCD)image acquisition module.The accuracy of temperature,pressure modules and CCD image recognition module are studied according to the strict requirement of the measuring device.3.The silicon sphere is placed in the mixture of 1,2,3-tribromopropane and 1,2-dibromoethane.The using of vibrating tube densitometer,calculates of thermal expansion coefficient of the working liquid.A method for calculating the compression coefficient of working fluid is designed,and the influence of the liquid compression is analyzed.4.The difference of the density of single crystal silicon is measured.Analyzing the measurement results and the uncertainty and evaluating the uncertainty.The relative standard uncertainty is 2.11×10-7degrees,indicating that the PFM has high density resolution.